A network system includes a plurality of nodes that transport data packets from a source computer to a destination computer. These network nodes exchange their routing information with each other to recognize what path (route) is used to deliver each packet to its destination. Routing information sent from a node indicates transmission paths that the sending node recognizes. Based on the routing information received from other nodes, each node selects forwarding directions according to the destination of individual packets.
A node may have a plurality of transmission paths to reach the destination computer. In that case, the node chooses one of them by using the algorithms specified in routing protocols such as Open Shortest Path First (OSPF) and Routing Information Protocol (RIP). Basically these routing protocols seek and select the shortest paths between nodes. For more sophisticated control, some existing routing protocols try to smooth out unevenness of traffic to minimize the peak network usage while seeking the shortest paths.
Data is transported over a network in the form of packets. With the conventional routing protocols noted above, the traffic of packets is forwarded along previously determined paths. Every node on the network has thus to be active even in a period when the overall amount of traffic is small.
With the enhancement of performance in network processing, the communication devices used in recent network systems consume more power than before. This leads to an increased importance of techniques for managing power consumption of a network system as a whole. For example, the dynamic voltage scaling (DVS) technique may be applied for the purpose of power conservation in routers and other network devices. DVS controls power consumption by varying supply voltage and operating clock. For example, the power supply voltage and operating clock frequency are lowered to reduce the power consumption when the device is under light load conditions.
Some recent routers are designed to go into a suspended state when the traffic is absent. This feature of routers may be used to save the power of a network. That is, the traffic on a network may be consolidated into a small number of paths, causing some routers to receive no incoming traffic to process. Those inactive routers are allowed to reduce their power consumption by entering a suspended state.
For example, there is a technique that ensures consolidation of network traffic into a limited number of paths. According to this technique, an alarm is raised when the amount of traffic on a link exceeds a specific threshold. The alarm information is transmitted from the source router of the traffic on that link to other routers, and the receiving routers change their packet forwarding mode from consolidation mode to distribution mode to distribute the traffic again to a plurality of paths. This technique prevents the traffic consolidation from causing congestion. See, for example, Japanese Laid-open Patent Publication No. 2010-206467.
It takes a long time, however, for nodes to recover from power-saving state to normal operation state to deal with an increased amount of traffic. This may lead to quality degradation such as transmission delays. The network system actually transports various groups of data with different tolerance levels for quality degradation. A problem arises when the quality degradation of some transmitted data exceeds its tolerance level.